Al–Sc Bonded Complexes: Synthesis, Structure, and Reaction with Benzene in the Presence of Alkyl Halide

An alumanyl anion possessing a new skeleton with N,N'-bis(2,6-diisopropylphenyl)-1,3-propanediamine ligand was synthesized and characterized. Transmetalation of this Al anion with diaminoscandium chloride precursors afforded the corresponding Al–Sc complexes possessing an unprecedented Al–Sc bond. The Al–Sc complex having Sc[N(SiMe3)2] unit underwent intramolecular C–H cleavage to form a bridged dinuclear complex with 𝜇-hydrido and 𝜇-methylene ligands. The Al–Sc complex having Sc(NiPr2)2 unit reacted with benzene in the presence of alkyl bromide to furnish a 1,4-dialuminated cyclohexadiene product with a concomitant formation of alkyl-alkyl coupled product. Although the latter product seems to form through the radical mechanism, DFT calculations revealed an ionic mechanism involving the following four steps: (1) C–Br bond cleavage through cooperative participation of both the Al and Sc centers, (2) C–C bond formation by a nucleophilic attack of the resulting alkyl-Al intermediate, (3) addition of Al anion moiety to benzene mediated by scandium center to form anionic Meisenheimer type intermediate, (4) subsequent reaction of the resulting Meisenheimer intermediate with bromoalumane intermediate to form dialuminated cyclohexadiene product. Such bimetallic reaction pathways to react with alkyl bromide and benzene would provide new insight into the chemistry of metal-metal bonded compounds.